a. Technical Field
The present invention relates to a light control sheet which constitutes a part of a surface light source suitably used as a backlight of a liquid crystal display etc., and a surface light source using the light control sheet.
b. Background Art
Surface light sources called edge light type backlights are conventionally used for liquid crystal displays and so forth. In general, in such surface light sources, a light reflecting sheet is laminated on a surface opposite to a light emerging surface of a light guide panel, at least one of which sides is provided with a light source of a cold-cathode tube or the like, and a light diffusing film and two or more upward prism sheets (prism sheets designed so that a prism surface should serve as a light emerging surface) are laminated on the light emerging surface of the light guide panel.
Further, in recent years, it is being attempted to develop surface light sources with so-called superior cost performance, in which the number of prism sheets laminated in a plural number is reduced to reduce the cost even with increasing front luminance (luminance for the direction of the normal of the light emerging surface of the light guide panel).
Examples of such surface light sources include those utilizing a prism sheet called downward prism sheet. Common prism sheets are designed so that the prism surface should serve as a light emerging surface. In contrast, the downward prism sheet is a prism sheet designed so that the prism surface should serve as a light entering surface.
Surface light sources utilizing such a downward prism sheet have a problem that, although the front luminance is increased, luminance for a direction of an angle slightly deviated from the front direction (direction of the normal of the light emerging surface of the light guide panel) is markedly reduced, and thus the viewing angle is narrowed, that is, the directivity of lights emerging from the surface light sources becomes unduly high. Therefore, a significant difference in brightness is caused between the central part and peripheral part of a screen depending on the parallactic angles of viewers.
Thus, a surface light source device is proposed by Patent document 1, in which the viewing angle is widened by disposing a light diffusing sheet on a surface of downward prism sheet opposite to the prism surface, and generation of iridescent ring pattern is avoided by forming convexo-concaves on the surface of the light diffusing sheet facing the downward prism sheet. However, with such a light diffusing sheet having a rough surface, when the rough surface contacts with the prism sheet, the rough surface damages the contacted surface, and therefore a problem is caused that display quality of a display apparatus using the surface light source device is degraded. For this reason, it is necessary to provide a gap between the prism sheet and the light diffusing sheet to secure a non-contact state, which inhibits manufacture of thinner surface light source devices.